Last year's runner-up Angelique Kerber claimed a remarkable comeback victory over wildcard Daria Gavrilova in a match finishing deep into the early hours of the morning at the Apia International.

Heavy rain in Sydney caused significant delays to play later in the day and held up this second-round clash in which fifth-seeded German Kerber eventually triumphed with a 6-7 (6-8), 7-6 (7-2), 6-3 win against the young Russian in over two and a half hours.

Kerber's triumph - in a match that finished after 3am local time - set up a quarter-final match against Garbine Muguruza, who successfully saw off 2013 winner Agnieszka Radwanska with a shock defeat.

Kerber wrote on Twitter: 'playing tennis all night long , finish my match at 3.10am!'

Number three Radwanska, who alongside Jerzy Janowicz helped Poland claim the Hopman Cup, won the first set against the Spaniard but then lost the next two as Muguruza hit back for a 3-6, 7-6 (7-4), 6-2 triumph.

A gracious Radwanska said of her opponent: 'I think she has really good potential.

'I think every day she's playing really good tennis, better tennis, and definitely the tennis that we are going to see much more of in the future.'

Spain's Garbine Muguruza hits a shot during her second-round victory against Agnieszka Radwanska

Top seed Simona Halep withdrew from the tournament ahead of her scheduled match with Karolina Pliskova on Tuesday due to gastrointestinal illness.

The Romanian, who was replaced in the draw by American lucky loser Nicole Gibbs, hopes to be healthy for the Australian Open, though.

She said: 'You never know with the stomach, but hopefully in two or three days I'll be ready to play again. Right now I just need to think about recovering, then I will worry about matches later.'

Third seed Radwanska suffered a three-set defeat to Muguruza at the Apia International

In on-court action, second seed Petra Kvitova enjoyed a relatively comfortable outing as she beat Shuai Peng 6-1, 7-5 but two of her fellow seeds fell by the wayside.

Sixth seed Ekaterina Makarova crashed out having won the first set as Carla Suarez-Navarro came back to record a 4-6, 6-1, 6-4 triumph.

Ninth seed Roberta Vinci could also have joined the list of second-round casualties but she fought back against Annika Beck to beat the German 3-6, 6-3, 6-3.

Barbora Zahlavova Strycova and Tsvetana Pironkova also won their matches.

Petra Kvitova acknowledges the crowd after winning her match against China's Shuai Peng

A general view of the stadium as the sun goes down over Sydney Olympic Park on Tuesday

In the men's event, Juan Martin Del Potro made a winning return to ATP Tour action after almost a year out as he defeated Sergiy Stakhovsky in straight sets.

The 26-year-old Argentinian has been sidelined since last February with a wrist injury but he showed little sign of rustiness as he began the defence of his Sydney title with a 6-3, 7-6 (7-4) victory.

Del Potro said: 'It was a great moment for me.

'I think I played well in my first match after 10 months. I served well. My forehand is still working out. So that's a good signal for the future.

'I need to work very hard on my backhands and my movements, but I think now is time to think about my comeback and enjoy this moment a lot.'

The 2009 US Open champion, who has dropped down to 338th in the world, will face top seed Fabio Fognini in the second round.

Del Potro added: 'I think the wrist is okay. I will be ready to play tomorrow for sure. I will do treatment; it takes me like an hour every day.'

Del Potro reaches out to his left to play a backhand during his first-round match in Sydney

Home hope Bernard Tomic, who was beaten by Del Potro in last year's final having won the title in 2013, also safely progressed to the second round as he swept aside Dutch qualifier Igor Sijsling 6-1, 6-2 in just 43 minutes.

Seventh seed Jeremy Chardy is also through, the Frenchman seeing off Sam Querrey 6-4, 6-4, but it was not such a good day for number eight Martin Klizan, who was dumped out in under an hour as Viktor Troicki recorded a 6-4, 6-4 win.

Janowicz was pushed to the limit by home hope Nick Kyrgios before emerging the winner in just over two hours.

Australian Bernard Tomic plays a backhand during his straight-sets victory against Igor Sijsling

The Pole, fresh from helping his country lift the Hopman Cup last weekend, breezed through the opening set but then lost the second on a tie-break to his 19-year-old opponent.

The deciding set also went the distance before Janowicz eventually wrapped up a 6-1, 6-7 (6-8), 7-6 (7-5) win.

Vasek Pospisil, Pablo Andujar and Gilles Muller were among the other players to progress.

Nick Kyrgios sits on his courtside chair and listens to music during a rain delay in Sydney on Tuesday